Wireless digital communication between a stationary base unit and one or more mobile wireless terminals is well known in the art. There are in general three types of transmission methods. In Frequency Division Multiple Access (FDMA), the available electromagnetic communication spectrum is divided into a plurality of frequency channels. Communication between the stationary base unit and one of the wireless terminals is affected over one of the frequency channels. Communication between the stationary base unit and a different wireless terminal is affected over a different frequency channel. In Time-Division Multiple Access (TDMA), transmission between a stationary base unit and a first wireless terminal is affected over a first "slice" in time. Transmission between the stationary base unit and a second wireless terminal is affected over a second "slice" of time, different from the first "slice". Finally, in Code Division Multiple Access (CDMA), communication between a stationary base unit and one or more wireless terminals is accomplished through spread spectrum transmission over a frequency range wherein a unique Pseudo Noise (PN) code distinguishes the communication between a stationary base unit and a first wireless terminal and a different PN code distinguishes the communication between the stationary base unit and a second different wireless terminal. See, for example, U.S. Pat. No. 5,267,244.
The introduction and wide spread use of personal computers in the last decade has given rise to the need for interconnection of these personal computers. This has resulted in the need to design and to develop Local Area Networks (LAN) to interconnect the PC-based processing units. The prevalent LAN's are: Ethernet, where the stations are all connected onto a cable; and Token Ring, where all the stations are connected to each other in the form of a ring. Ethernet is based upon the concept of carrier sensing where all stations listen to the cable and only access the medium if it is idle. In Ethernet, however, since all the competing stations can access the medium at the same time, collision is possible. Thus, Ethernet must provide for collision detection and retry. Token-ring is based on each of the stations waiting for an access token which is passed in round-robin fashion. Since access to the transmission medium occurs only upon the receipt of the token, collision is avoided. However, the stations do have to wait for their assigned token.
With the introduction of portable laptop and notebook computers and the merging of the computer with the wireless medium, such as radio frequency (as exemplified in cellular telephone networks), or infrared waves or other signals, there is a growing need for wireless connectivity for these portable computers. Wireless LAN's face a number of hurdles. These include restrictions on the allocation of frequency spectrum, limitations on the feasible data rates over the air, interference propagation, and power consumption.
Furthermore, protocol such as Ethernet or token-ring are inadequate for wireless LANs for several reasons. First, with respect to Ethernet, it assumes that all stations can listen to one another, which may not be possible in the wireless case. Secondly, due to the introduction of multi-media applications, there is an additional requirement to support both asynchronous services (e.g. data) and time based services (e.g. voice).
One prior an solution to the first problem, which is also known as he "hidden terminal" problem is to use a hub-based system, wherein all information exchanged between two wireless terminals is relayed by a hub or central stationary base unit. This is a waste of precious wireless bandwidth when the two wireless terminals are in range of one another. With respect to the second problem, the prior an has suggested partitioning the medium into two bands: one for asynchronous service and the other for time-based services, and providing independent protocol and controls for each of the two bands. This has now been codified in the United states by the FCC in its recent ruling on unlicensed Personal Communication Service (PCS). Asynchronous service requires the entire medium bandwidth for a burst duration and are tolerant to variable access delays, whereas time-based services require a fraction of the bandwidth on a periodic basis and are not tolerant of variable access delays.